1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine or, in particular, to a control apparatus, for an internal combustion engine, aimed at improving the stability of the idle state by improving the combustion immediately after a cold start of the engine.
2. Description of the Related Art
Conventionally, the fuel injection amount and the fuel injection timing of an internal combustion engine (hereinafter referred to simply as "the engine") are determined by the engine load which in turn is determined based on the intake air amount of the engine and the engine speed. The fuel injection timing is set generally at a time point before the intake valve of each cylinder opens, except at a transient time when the load undergoes a sudden change. This fuel injection is called intake-asynchronous injection.
In this conventional technology, however, the fuel injected in an intake-asynchronous mode at the time of an engine start, especially, at the time of an engine cold start, sometimes fails to vaporize, attaches to the surface of the intake pipe and is undesirably supplied to each cylinder in liquid form. In such a case, the problem of a deteriorated emission is caused by the increase of such components as CO and HC in the exhaust gas.
To prevent this inconvenience, Unexamined Patent Publication No. 3-23342 and Unexamined Utility Model Publication No. 61-82050 propose a method of preventing the deterioration of the combustion and hence the deterioration of the emission by injecting the fuel synchronously with the intake timing when starting the engine at a low temperature. The intake-synchronous injection is defined as synchronizing the fuel injection timing with the opening of the intake valve of the cylinder into which the fuel is to be injected.
According to the technology disclosed in Unexamined Patent Publication No. 3-23342 and Unexamined Utility Model Publication No. 61-82050, the engine is always switched to the intake-synchronous injection at the time of a cold start. In the case where the switching to the intake-asynchronous injection is not required, such as when the engine is in satisfactory combustion state, however, the emission (exhaust HC) is undesirably increased. This is because, in intake-synchronous injection, the fuel is generally supplied to the combustion chamber in a state not mixed well with the intake air, and the delayed fuel atomization results in an increased amount of emission.
On the other hand, the current trend is toward the practice of changing the components of the gasoline constituting the fuel from one season to another. Specifically, the practice is spreading to supply a light fuel having a superior combustion property in winter when the temperature is low and to supply a heavy fuel, which is hard to vaporize, in summer when the temperature is high. An engine, which remains in the intake-asynchronous injection mode but not in the intake-synchronous injection mode at the time of cold start, may fail to start when supplied with a heavy fuel. The conventional practice, therefore, is to effect the intake-synchronous injection for a predetermined length of time at the time of cold start in order that the engine can be always started regardless of the fuel type. In the case where the intake-synchronous injection is always effected for a predetermined length of time at the time of cold start of an engine supplied with light fuel, however, the emission is deteriorated at the cold start time as described above.